It is known to produce chlorine dioxide by reduction of an acid aqueous chlorate solution using various reducing agents such as methanol, chloride ions, hydrogen peroxide, etc. In U.S. Pat. No. 2,833,624 (Sprauer), a process is described whereby chlorine dioxide is produced by reacting chlorate ions, a strong acid and hydrogen peroxide. The process can be carried out in the presence of chloride ions, which are said to catalyze the reaction. U.S. Pat. No. 2,833,624, however, does not teach carrying out the process in a single vessel under subatmospheric pressure.
U.S. Pat. No. 4,421,730 (Isa et al) also discloses a process to produce chlorine dioxide using hydrogen peroxide. This process, however, requires the reduction to be effected using chloride ions in the presence of a complex palladium (II) catalyst and the hydrogen peroxide in order to achieve commercially attractive production rates.
Recently, there issued U.S. Pat. Nos. 5,091,166 (Engstrom et al) and 5,091,167 (Engstrom et al) wherein chlorine dioxide is described as being produced in a single-vessel generator-evaporator-crystallizer process using hydrogen peroxide as the reducing agent. Hydrogen peroxide for use in the process generally is employed in its commercially-available form, generally an aqueous solution containing about 30 wt % hydrogen peroxide or higher. Alkali metal chlorate, hydrogen peroxide and sulfuric acid are continuously fed to the generator. Commercially acceptable production rates and efficiencies are said to be obtained. The main distinction between both processes is that U.S. Pat. No. 5,091,166 covers the acidity range 2 to 5N whereas U.S. Pat. No. 5,091,167 covers the acidity range 5 to 11N.
The operation of a hydrogen peroxide-based chlorine dioxide generator using one of the processes described in U.S. Pat. Nos. 5,091,166 and 5,091,167 generally leads to satisfactory results in terms of the chemical yield of chlorine dioxide on chlorate. These processes, however, have certain deficiencies and disadvantages which can seriously affect the operation of the generator. For example, there can occasionally be an excessive consumption of hydrogen peroxide, leading to inefficient operation. Also there can be excessive foaming of the liquor in the generator, potentially leading to unsafe operation and uneven production rates of chlorine dioxide. Additionally, the process can have high sensitivity to metallic impurities (such as iron) inadvertently but often present in variable and uncontrollable amounts in the liquor, there can be occasional variations in the purity of the product, there is enhanced corrosion of the equipment, etc. In other words, continuous operation of the processes in U.S. Pat. Nos. 5,091,166 and 5,091,167 can, on occasion, become highly inefficient and unpredictably unstable, leading to potentially unsafe operation.